White light emission from fluorene-EDOT and phenothiazine-hydroquinone based D-π-A conjugated systems in solution, gel and film forms

Article abstract of DOI:10.1039/c7nj01064h

Two conjugated D-π-A organic molecules, one fluorene-ethylene dioxythiophene (FL-E) based and another phenothiazine-hydroquinone (PT-Hq) based, were synthesized and observed to form 1-dimensional microstructures in the solid state. These two molecules whe

Full text of DOI:10.1039/c7nj01064h

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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White light emission from fluorene-EDOT and phenothiazine-
hydroquinone based D-π-A conjugated systems in the solution,
gel and film forms
Received 00th January 20xx,
Accepted 00th January 20xx
Vivek Anand and Raghavachari Dhamodharan*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Two conjugated D-π-A organic molecules, one fluorene-ethylene dioxythiophene (FL-E) based and another phenothiazine-
hydroquinone (PT-Hq) based were synthesized and observed to form 1-dimensional microstructures in solid state. These
two molecules when mixed with rhodamine B dye (Rh-B) and photoexcited at 411 nm, generated white light with
Commission Internationale d’Eclairage (CIE) coordinates (0.32, 0.33) in solution state. Similarly, white light was also
obtained in the poly(methyl methacrylate) film (0.30, 0.33) and in gelatine gel (0.31, 0.34) for the same composition of the
mixture as in the solution state. The correlated colour temperatures obtained in the three phases (6111 K in solution, 6900
K in PMMA film and 6400 K in gelatin gel) suggest that the system emits cool white light. In order to verify the D-π-A
nature of the molecules, intramolecular charge transfer studies were done, which revealed that both the molecules
showed positive solvatochromism. The plausible mechanism behind white light emission is Forster resonance energy
transfer accompanied by simultaneous emission. This was verified through fluorescence titration experiment, lifetime
studies, spectral overlap and Forster distance calculations and correlated to results from cyclic voltammetry and DFT
studies. This is the first report on the emission of white light from D-π-A system by FRET.
of their electronic functions, are of immense importance in
optoelectronic fields such as organic light emitting diodes
(OLEDs), field effect transistors (OFETs) and solar cells.13-15 The
Introduction
White light emitting devices, specifically white light emitting
diodes (WLEDs) have gained enormous importance due to
their potential applications in the fields of full colour panel
displays, liquid crystal displays and back lighting sources.1-5
Organic materials owing to their low cost, light weight,
flexibility, ease of synthesis and the ability to form ultra-thin
films are preferred over their inorganic counterparts in white
light applications.6,7 White light in organic materials can be
achieved either by blending of different components
(simultaneous emission of the three primary colours blue,
green, and red) or from a single organic moiety, fluorescence
of which, covers the entire visible region.8-10 However, due to
the constraint imposed by Kasha’s rule, it is very difficult for a
single organic material to emit white light. Hence, multi
component organic white light emitting materials are in high
demand.11,12
solution processability and choice of molecular design give
extra edge to their applicability. Hence, implication of such
donor-acceptor type molecules in white light emission can be
of great use.
Although organic conjugated molecules have been used for
white light emission, not many D-A type molecules have been
exploited for the same.16-18 For instance, in 2013, Baumgartner
et. al. reported white light generation based on donor-
acceptor type dithienophosphole. In the same year, Wang et.
al. reported two weakly interacting donor-acceptor dyads
exhibiting white photoluminescence through controlled acid
protonation.18 In both the reports, white light was obtained
using concentrated acids viz. camphorsulfonic acid and
trifluoroacetic acid, which are highly corrosive. It is important
that such harsh conditions are avoided in the generation of
white light. 1D nano- and micro- materials are more promising
building blocks for optoelectronic devices as compared to zero
dimensional (0-D) materials and in this context D-A type
molecules that form one-dimensional (1-D) microstructural
morphology are sought after.19-21 Even though, solution phase
white light generation is more common, the development of
white light in gel and film form are of more applicability in
optoelectronic devices.22,23
Donor-acceptor (D-A) conjugated organic molecules by virtue
Department of Chemistry, Indian Institute of Technology Madras, Chennai-
600036, India; Email: damo@iitm.ac.in
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Several approaches have been used in producing white light
viz. single molecular excited state intramolecular proton
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transfer (ESIPT),24,25 solvent assisted self-assembly,22,26,27 obtained by the bromination of (
Journal Name
8
), which in turn, was
ESIPT coupled aggregation induced enhanced emission obtained by the formylation of (
(AIEE)28,29 and fluorescence resonance energy transfer
). Then, bromoaldehyde
(FRET).30-33 FRET is a phenomenon in which a donor moiety in dialkynedioctyloxybenzene (
) to give PT-Hq via Sonogashira
7
), preceded by alkylation of
(
6
(9) was reacted with
5
its excited electronic state transfers its energy to an acceptor coupling reaction (Please see the detailed steps followed in the
moiety. Conditions necessary for FRET to take place are synthesis in Schemes S1). The structure of PT-Hq was
appreciable overlap between the fluorescence spectra of the characterized by the spectroscopic tools and found to be
donor and the absorption spectra of the acceptor. consistent with that expected (elaborate spectroscopic
Furthermore, the distance between the donor and the characterization and assignments are given in the supporting
acceptor should be small enough for energy transfer to take information Figs. S2-S7). The structures of blue, green and
place.33,34
orange emitting fluorescent moieties viz. FL-E
,
PT-Hq and Rh-
In the above context, the use of non-harsh conditions and
white light generation in all the three forms (viz. solution, film
and gel) acquire significance. In this work, two conjugated D-π-
A organic molecules were designed and synthesised; one
fluorene and ethylenedioxythioiphene (EDOT) based (FL-E) and
another phenothiazine and hydroquinone based (PT-Hq). The
synthesis of PT-Hq is new while we reported the synthesis of
FL-E recently.35 The reasons for choosing the specific fused
aromatic ring systems are: fluorene is a good hole transporter,
has excellent thermal and chemical inertness as well as good
solubility and high fluorescence quantum yield.36-38 EDOT, on
the other hand, exhibits high molar absorption coefficient (ε)
and due to the presence of heteroatoms efficiently donates
electrons.39-43 Phenothiazine was chosen because of its non-
planar puckered structure and high fluorescence.44,45 The
motivation behind the synthesis is that when FL-E, a blue
B
, respectively, are shown in Scheme 1.
S
S
CHO
OHC
C10H21
C10H21
O
O
O
O
C12H25
N
FL-E (a)
OC8H17
S
CHO
OHC
S
N
OC8H17
C12H25
em.
emitter (λmax = 440 nm) is mixed with PT-Hq, which is a
em.
PT-Hq (b)
green emitter (λmax
= 517 nm) and an orange coloured
rhodamine B dye (λmaxem. = 579 nm), white light generation can
be expected. Interestingly, due to aggregation defect, green
emission was observed for FL-E in solid state (the solid state
fluorescence spectrum of FL-E is shown in Fig. S1). Infact, the
dialkylfluorene are known to show green emission in the solid
state due to keto or aggregation defects.46-48 The details of the
emission and its mechanism were thoroughly examined by
fluorescence titration, spectral overlap studies, fluorescence
lifetime studies, electrochemical studies and DFT calculations.
In order to establish the D-π-A nature of the molecules,
intramolecular charge transfer (ICT) studies were also done.
Moreover, the emission of white light in poly(methyl
methacrylate) [PMMA] film and gelatine gel forms are also
obtained.
N
O
N+
COOH
Rh-B (c)
Scheme 1: (a-c) Chemical structures of FL-E, PT-Hq and Rh-B, respectively.
Morphology
In the solid state, FL-E and PT-Hq formed 1D microwires as
shown in Figure 1(a) and 1(b), respectively. The diameter of
the microfibers obtained in the case of FL-E is 300-350 nm,
while that in the case of PT-Hq is 150-200 nm. Microfibrous
alignment was also obtained in hexane, THF and DMF solution
for PT-Hq. The attainment of microfibers in solution and solid
phase depicts the strength of efficient π-π stacking, despite
the presence of long alkyl chain, which is not very common in
the case of organic molecules. This property could enable their
use in nanolaser and other nano- and micro- optical devices.49
Results and discussion
Synthesis
The synthesis of FL-E was reported by us recently in another
context.35 For the synthesis of PT-Hq (for complete numbering
of the molecules, please refer to Scheme S1),
dialkynedioctyloxybenzene
Sonogashira reaction from 1,4-diiodooctyloxybenzene (
1,4-diiodooctyloxybenzene (
of dioctyloxybenzene ( ), which in turn was synthesized by the
(
5
)
was
synthesised
via
ICT studies
3
). The
3
) was synthesised by iodination
In order to confirm the D-π-A nature of the molecules PT-Hq
and FL-E, intramolecular charge transfer studies were done.
2
alkylation of hydroquinone (1). The bromoaldehyde (9) was
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White Light Emission (in Solution, Film and Gel Phases)
The absorption spectra of FL-E (in THF), PT-Hq (in THF) and Rh-
(in water) showed absorption maxima at 406 nm, 411 nm
and 553 nm, respectively. While the emission maxima for FL-E
B
,
PT-Hq and Rh-B are at 440 nm, 517 nm and 579 nm,
respectively. The UV-visible and fluorescence spectra of each
of the materials are shown in Figure 3. Inset shows the
photographs of the corresponding materials in solution state
under UV light.
10 µm
Figure 1. SEM images of (a) FL-E and (b) PT-Hq both showing fibrous alignment
The emission spectra of PT-Hq in nine different solvents with
different polarities were recorded as shown in Figure 2(a). It
was found that from hexane (λmax = 492 nm) to THF (λmax = 520
nm) there was a red shift of 28 nm in the emission λmax
.
Furthermore, from THF to DMSO (λmax = 543 nm) there was
again a red shift of 23 nm in the λmax. This red shift was
accompanied by decrease in fluorescence intensity. These
results suggest that the molecule shows positive
solvatochromism.50-52 Moreover, in order to get insight into
the ICT nature, the emission energy versus solvent polarity
study (Lippert-Mataga plot) was carried out as shown in Figure
2(b). A plot of the solvent polarity versus maximum emission
energy (cm-1) gave a straight line. The linear nature of the
curve shows that only one excited state is present.53,54
Similarly, in the case of FL-E a red shift of 35 nm was observed
in the λmax of the fluorescence spectrum as the polarity is
changed from hexane to DMF. The spectrum is shown in Figure
S8. Thus, both the molecules showed ICT properties, which
confirm their D-π-A nature.
em.
λmaxabs.). Inset shows the photographs of their respective solutions under UV light
irradiation
Figure 3
.
UV-Visible and fluorescence spectra of FL-E
,
PT-Hq and Rh-B (λexc.
=
When 1 mL of FL-E (in THF) was mixed with 1 mL of PT-Hq (in
THF) and 1.4 mL of Rh-B (in water), each of initial
concentrations 10-5 M, and excited with UV light of wavelength
411 nm, the mixture exhibited two fluorescence bands at 477
Figure 2. (a) ICT studies of PT-Hq in different solvents (b) Lippert-Mataga plot for
PT-Hq
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nm and 585 nm, respectively, as shown in Figure 4(a). It also corresponding to this mixture is shown in Fig. S11. The CIE
emitted white light with a commission Internationale de coordinates for this emission (0.30, 0.29) does not correspond
L’eclairage (CIE) coordinates of x = 0.32, y = 0.33, which is to pure white light (CIE diagram is shown in Fig. S12).
almost equal to the standard CIE for white light viz. (0.33, 0.33) Moreover, no white light was obtained in gel or film form by
and correlated colour temperature (CCT) of 6111 K, as shown the combination of these two moieties in the same proportion
in Figure 4(b).
as in solution. In another control experiment, FL-E and PT-Hq
were mixed (1 mL each of initial concentration 10-5 M) and
excited with UV light of wavelength 411 nm. The CIE obtained
was (0.20, 0.28) and does not correspond to white light
emission. The results from this study are shown in Fig. S13.
Similarly, PT-Hq and Rh-B were mixed (1 mL each of initial
concentration 10-5 M) and excited at the wavelength of 411
nm, to give a greenish-orange light with CIE coordinates (0.44,
0.52). This data is presented in Fig. S14. Thus, no efficient
white light was obtained by the mixing of any two of these
molecules. The mixing of all three components in the mole
ratio 1:1:1.4 is a prerequisite for the attainment of efficient
white light as confirmed by the test experiments.
Figure 4.
(a) Emission spectrum of mixture of FL-E (1 mL of 10-5 M, in THF), PT-Hq
(1 mL of 10-5 M, in THF) and Rh-B (1 mL of 10-5 M, in water) in solution state,
excited at 411 nm. Inset shows the photograph of corresponding white light
under UV excitation. (b) CIE diagram of the corresponding white light emission in
solution state.
The effect of change in the concentration of the molecules on
white light emission was studied. At first, the concentration of
FL-E and PT-Hq were kept constant at 1 mL (10-5 M each) and
the concentration of Rh-B was increased in aliquots of 0.2 mL
(10-5 M). At 1:1:1 ratio of FL-E:PT-Hq:Rh-B, the emission
intensity due to the peak at 585 nm (mainly due to Rh-B) was
not equivalent to the peak around 477 nm. The initial results
suggest that the value of spectral overlap between FL-E and
PT-Hq was 7.6 × 1014 M-1 cm-1 nm4, which is higher than that
for PT-Hq and Rh-B (4.88 × 1014 M-1 cm-1 nm4). It was also
evident from the quantum yield values that significant energy
transfer was taking place from the blue to green component
(ΦPL = 0.48) as compared to the green to red component (ΦPL
=
0.18). Hence, the intensity mismatch was probably due to the
lesser energy transfer from PT-Hq to Rh-B as compared to that
from FL-E to PT-Hq. In order to achieve the pure white light
emission (0.33, 0.33), the concentration of the red component
was increased. At higher concentration, the emission intensity
of the red component was increased as a result of partial
energy transfer from FL-E followed by PT-Hq. Hence, 1:1:1.4
molar ratios was the optimized value to get pure white light.
The CIE diagram with changing concentration of Rh-B, keeping
FL-E and PT-Hq constant is shown in Fig. 5a, while the
fluorescence spectra is shown in Fig. 5b. Similarly, the
concentration of PT-Hq was varied, keeping the concentration
of Rh-B and FL-E constant, the results of which are shown in
supporting information Fig. S9. Then, the concentration of FL-E
was varied, keeping PT-Hq and Rh-B constant, the results of
which are shown in supporting information Fig. S10. These did
not lead to appropriate CIE. Thus, the intensity match of the
two peaks is critical for efficient white light emission. This
intensity match depends on energy transfer and hence on
spectral overlap between the molecules, which eventually
leads to efficient white light emission.34
Figure 5. (a) CIE chromaticity diagram obtained on changing the volume of Rh-B
from 1 mL to 1.8 mL, while keeping the volume of FL-E and PT-Hq at 1mL each.
(b) The corresponding fluorescence titration spectra.
The emission of the mixture was studied in the film form
(PMMA) and the result is shown in Fig. 6(a). The film showed
two fluorescence bands at 494 nm and 590 nm. The CIE
coordinate obtained is (0.30, 0.33) [the CIE diagram is shown
in Fig. 7(a)] with a CCT of 6900 K. This means that the film
emits cool white light. The white light emission was quite
stable even after two weeks of the formation of film. Although
the colour of the film was pale pink under the normal light, it
showed white light when illuminated by UV light as shown in
the inset of Fig. 6(a). In order to further broaden its utility, the
In control experiments, FL-E and Rh-B (1 mL each of initial
concentration 10-5 M) were mixed. The emission spectrum
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emission of light was investigated in the gelatin gel phase. In
this case, two fluorescence bands were observed at 471 nm
and another at 581 nm, respectively, as shown in Fig. 6(b). The
peak around 471 nm is quite broad and may be because of
the combination of peaks due to the mixing of FL-E and PT-Hq
.
In gel, the CIE coordinates obtained was (0.31, 0.34) [CIE
diagram is shown in Fig. 7(b)] and the CCT of 6400 K was
obtained. Thus, white light was obtained with excellent CIE in
all three phases viz. solution, PMMA film and gelatin gel. The
emission was quite persistent and did not lose its white colour
even after two weeks. The CCT obtained in all three cases hints
towards cool white light emission. The attainment of white
light in gel as well as film form enhances its applicability as
solid state emitters are always preferred over their solution
phase counterparts, from device fabrication point of view in
optoelectronic applications.
Figure 7. (a) CIE diagram of the corresponding white light emission in PMMA
film. (b) CIE diagram of the corresponding white light emission in gelatin gel
E is the efficiency of energy transfer, τD is the lifetime of the
donor in absence of acceptor. The distance between donor
and acceptor was calculated by the formula:
ꢇο
r =
(2)
ꢋ
ꢉ ꢊ ꢅ
ꢈ
ꢆ
Rο is the Forster distance, the distance at which rate of
energy transfer is 50 %. The Forster distance was calculated by
the formula:
Rο = 0.211[κ2 × n-4
×
Ԛ
D × J(λ)]1/6
(3)
where, κ2 is the orientation factor (the value of κ2 is taken as
2/3), n is the refractive index of the medium, ԚD is the
quantum yield of the donor in the absence of acceptor and J(λ)
is the spectral overlap.
The spectral overlap J(λ) was calculated using the formula:
ꢜ ꢔꢕ ꢖ ꢗꢘ ꢖ ꢖ ꢚꢁꢛꢄ
ꢙ
ꢁ
ꢄ
ꢁ ꢄ
ꢌ
J(λ) = ꢒ Fꢍ ꢎ ꢏꢐ ꢎ ꢎ dꢁꢎꢄ =
(4)
ꢝ
ꢑ
ꢁ ꢄ ꢁ ꢄ
ꢌ
ꢜ ꢔꢕ ꢖ ꢚꢁꢛꢄ
ꢓ
ꢁ
ꢄ
ꢌ
ꢝ
where, FD(λ) is the corrected fluorescence intensity of the
donor in the wavelength range of λ to λ + Δλ with the total
intensity (area under the curve) normalized to unity and εA is
molar extinction coefficient of the acceptor.
The transfer energy was calculated using the formula:
E = 1 - ꢅ
(5)
ꢆꢞ
ꢅꢆ
where, τDA is the lifetime of the donor along with the acceptor
and τD is the lifetime of the pure donor.55
In order to investigate the mechanism of white light emission,
the absorption and emission spectrum of the individual
components were compared. This reveals that there is
sufficient overlap between the emission spectrum of the donor
to the absorption spectrum of the acceptor molecules as
shown in Figure 8. The quantum yields for the emission of FL-E
and PT-Hq were measured using fluorescein and anthracene,
respectively, as standards in THF. The quantum yields for the
emission are found to be 0.43 and 0.57 for FL-E and PT-Hq,
respectively (the formula used to calculate quantum yield is
given in eq. S1). The quantum yield for the emission of
rhodamine B, as obtained from literature is 0.31 in water.56
To investigate the mechanism of white light emission
further, the spectral overlap J(λ), Forster distance between the
Figure 6. (a) Emission spectrum resulting from the mixture of FL-E (10-5 M, in
THF), PT-Hq (10-5 M, in THF) and Rh-B (10-5 M, in water) in PMMA film, excited at
411 nm. Inset shows the photograph of corresponding white light under UV
excitation. (b) Emission spectrum of mixture of FL-E (10-5 M, in THF), PT-Hq (10-5
M, in THF) and Rh-B (10-5 M, in water) in gelatine gel, excited at 411 nm. Inset
shows the photograph of corresponding white light under UV excitation.
Deducing the Mechanism
Calculation of spectral overlap J(λ), rate of energy transfer k
and distances r, Rο:
The rate of energy transfer was calculated using the formula:
ꢀ
k =
(1)
ꢁꢂꢃꢀꢄꢅꢆ
where,
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donor and acceptor were assessed. A significant overlap 9(b). There was also a gradual red shift in the emission peak of
between the emission spectra of FL-E and the absorption Rh-B from 584 to 590 nm. The initial sudden decrease in the
spectra of PT-Hq was observed in the solution state as shown intensities of PT-Hq may be due to the gradual addition of
in Figure 8(a). In fact, the spectral overlap J(λ) for the system water, as water is a quencher of the fluorescence of PT-Hq as
was found to be 7.6 × 1014 M-1 cm-1 nm4 as calculated using eq. verified by ACQ experiments done for PT-Hq
.
The
4. The Forster distance between the donor FL-E and acceptor photoluminescence spectra of PT-Hq in THF/water mixture
PT-Hq, as calculated from eq. 3 is 4.11 nm, which is within the with 0 to 90 % water fraction is shown in Fig. S15. It can be
expected range for smooth RET. A good spectral overlap seen that as the concentration of water was increased, there
between the emission spectrum of PT-Hq and the absorption was a continuous decrease in fluorescence intensity. To
spectrum of Rh-B was also observed as shown in Figure 8(b). further confirm the FRET mechanism, to a solution of FL-E in
The spectral overlap J(λ) was calculated using eq. 4 and was THF (1 ml of 10-4 M), Rh-B (in water) was added such that
found to be 4.88 × 1014 M-1 cm-1 nm4. The Forster distance was there was a gradual increase in the concentration of Rh-B
found to be 4.08 nm (eq. 3). The rate of energy transfer was while that of FL-E was constant (10-5 M). In this case also, a
also determined using eq. 5 and eq. 1, and was found to be decrease in the intensity of blue component with the gradual
12.4 × 107 s-1. Finally, the distance between the donor and the increase in the intensity of red component was observed on
acceptor was calculated using eq. 2 and was found to be 4.5 addition of each 1 μM concentration of Rh-B as shown in
nm. The emission spectrum of FL-E was also observed to Figure 9(c), along with the red shift in the λmax of Rh-B from
overlap significantly with the absorption spectrum of Rh-B as 587 nm to 592 nm. The signature of FRET is simultaneous
shown in Figure 8(c). The spectral overlap integral J(λ) was increase in the fluorescence intensity of acceptor with the
calculated to be 2.94 × 1014 M-1 cm-1 nm4, although the value is gradual decrease in the intensity of donor on addition of
less than that for FL-E
& PT-Hq and PT-Hq & Rh-B pairs. The increasing concentration of acceptor to a fixed concentration
essential conditions for FRET to take place are: appreciable of donor. This is clearly evident in all the three cases.
overlap between the fluorescence spectra of the donor and
the absorption spectra of the acceptor; and the distance Lifetime studies
between the donor-acceptor chromophore (Forster distance)
should be between 1-10 nm. All the values obtained in this To further verify if FRET could be operational, the fluorescence
case agree with the above mentioned criterion and hence lifetimes of PT-Hq and PT-Hq along with Rh-B in solution phase
strongly correspond to the possibility of FRET.
were recorded. The average lifetime of the mixture of PT-Hq
and Rh-B was found to be 2.66 ns, while that for pure PT-Hq
was 3.98 ns (as shown in Fig. 10). Clearly, there is a decrease in
the lifetime of the mixture as compared to pure PT-Hq. All
Fluorescence Titration
The change in the fluorescence intensity of the donor these observations lead to the conclusion that FRET is
molecule in the presence of the acceptor molecule responsible for the energy transfer taking place from FL-E to
(fluorescence titration) was studied to investigate the PT-Hq to Rh-B. Interestingly, there was energy transfer taking
mechanism of emission further. To a solution of FL-E in THF (1 place directly from FL-E to Rh-B. The FRET mechanism is
mL of 10-4 M), PT-Hq was added such that there was a gradual illustrated elaborately in Scheme 2.
increase in the concentration of PT-Hq from 1 μM to 6 μM,
keeping the concentration of FL-E constant (For example, at Electrochemical studies
first 1 mL each of 10-4
M FL-E was taken in 7 cuvettes; then in
the first cuvette 9 mL of THF was added to give a solution of The electrochemical behaviour of the molecules FL-E (in DCM),
10-5 M solution of FL-E. In the second cuvette, 8 mL of THF was PT-Hq (in DCM) and Rh-B (in water) was investigated in
added along with 1 mL of 10-5
M
PT-Hq, to give 10-5
M
FL-E and solution state as shown in Figure 11. The CV of the molecules
1 μM PT-Hq. Similarly, other solutions were made.). This envisaged an oxidation process with Eonset oxidation at 1.13,
resulted in a decrease in fluorescence intensity of the donor 0.62 and 0.15 V for FL-E PT-Hq and Rh-B, respectively. In the
,
moiety with the concomitant increase in fluorescence intensity case of PT-Hq, the oxidation process was reversible unlike FL-E
of the acceptor as shown in Fig. 9(a). For each 1 μM addition of and Rh-B, where irreversible oxidation took place. The onset of
PT-Hq solution, there was a decrease in fluorescence intensity oxidation peak can be attributed to the formation of radical
at around 440 nm, while there was a gradual increase in cation in the molecules. The irreversible reduction peaks were
fluorescence intensity at around 517 nm. There was also a obtained at -0.98, -1.23 and -0.92 V for FL-E, PT-Hq and Rh-B,
gradual red shift in the emission peak due to PT-Hq from 509 respectively. The redox potential of Fc/Fc+ (standard value 4.8
nm to 517 nm. This observation suggests that energy transfer eV with respect to vacuum) was observed at 0.33 V in 0.2 M
is taking place through FRET. Similar experiment was done terabutylammoniumhexafluorophosphate/DCM
solution.
starting from PT-Hq in THF to which Rh-B (in water) was added Based on this, the HOMO and LUMO energy levels of the
in lots 2 μM for each addition. Here again, there was a molecules were estimated using the equation.57
decrease in fluorescence intensity of donor moiety at around
517 nm, while there was a gradual increase in fluorescence
intensity of the acceptor at around 590 nm as shown in Figure
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ARTICLE
Figure 8. (a) Spectral overlap of fluorescence spectra of FL-E (blue) and absorption spectra of PT-Hq (red); (b) Spectral overlap of fluorescence spectra of PT-Hq (blue)
and absorption spectra of Rh-B (red); (c) Spectral overlap of fluorescence spectra of FL-E (blue) and absorption spectra of Rh-B (red).
Figure 9
with the addition of Rh-B (in water) excited at 411 nm. (c) Fluorescence spectra of FL-E (10-5 M, in THF) with the addition of Rh-B (in water) excited at 411 nm
.
(a) Fluorescence spectra of FL-E (10-5 M, in THF) with the addition of PT-Hq (in THF) excited at 411 nm. (b) Fluorescence spectra of PT-Hq (10-5 M, in THF)
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Journal Name
using the formula 1240/
λ
onset. The values found are 2.14,
2.03 and 1.72 eV, respectively. The solid state UV-visible
spectra of the compounds are shown in Fig. S16. Thus,
energy band gaps decreases from FL-E to PT-Hq to Rh-B
This indicates that energy transfer may be possible
between FL-E and PT-Hq and also between PT-Hq and Rh-B
.
.
Hence, electrochemical studies are also in agreement with
the possibility of FRET and also enable better understanding
of the positions of energy levels in the molecules.
Figure 10. Fluorescence decay curves of pure PT-Hq (10-5 M in THF) and PT-
Hq with Rh-B (10-5 M in water)
Scheme 2: Schematic presentation of FRET for the molecules FL-E, PT-Hq and
Rh-B in solution along with their approximate energy level diagrams.
EHOMO = - (EOX + 4.47) eV
ELUMO = - (Ered + 4.47) eV
(6)
(7)
where, EOX and Ered are the onset potentials of oxidation
and reduction, respectively, as measured against Ag/AgNO3
reference for FL-E and PT-Hq, while Ag/AgCl electrode was
used as reference for Rh-B. The HOMO energy levels of the
molecules are -5.60, -5.09 and -4.62 eV for FL-E
Rh-B respectively. Thus, the HOMO levels decrease
consecutively from blue (FL-E) to green (PT-Hq) to red (Rh-
), as expected. The LUMO levels, on the other hand, are -
3.49, -3.24, -3.55 eV for FL-E PT-Hq and Rh-B, respectively.
, PT-Hq and
,
B
,
The energy band gaps were calculated by subtracting the
LUMO energy levels from HOMO energy levels. The
electrochemical band gaps are found to be 2.11, 1.85, 1.07
Figure 11. Cyclic voltammograms of FL-E
0.2 M Bu4NPF6 as supporting electrolyte for FL-E and PT-Hq while
as supporting electrolyte for Rh-B.
,
PT-Hq and Rh-B in solution state,
0
.1 M KNO3
eV for FL-E, PT-Hq and Rh-B, respectively. Furthermore,
when the electrochemical band gaps were compared with
those of optical band gaps, which were calculated from the
solid state UV-visible spectra of the compounds, the values
obtained were similar. The optical band gap was calculated
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DFT studies
ARTICLE
Gas phase density functional theory (DFT) calculations
(B3LYP/6-31G* level) were carried out to get insight into the
electronic distribution and to study the chances of energy
transfer between the molecules. The DFT generated HOMO
and LUMO levels were also calculated for all the three
molecules. The HOMO, LUMO pattern are shown in Figs.
12(a)-12(c).
In the case of FL-E, the HOMO is located on fluorene and on
the sulphur atom of EDOT, while LUMO and LUMO + 1
degenerate orbitals are located on the oxygen atoms of
EDOT (LUMO + 1 orbital is shown in Fig. S17). Oxygen being
most electronegative carries most of the negative charge.
Thus, the HOMO and LUMO levels are separated and
localized. The HOMO energy level is 6.02 eV, while LUMO
energy level is 3.71 eV and the energy band gap is 2.31 eV.
This value is characteristic of blue emitters. On the other
hand in PT-Hq, the HOMO-LUMO is uniformly delocalized
throughout the molecule. This is common in case of
molecules, which quickly deactivate after excitation and
hence are highly emissive.58 In fact, PT-Hq is highly emissive
in the green region of the electromagnetic spectrum.
Moreover, the theoretically calculated HOMO energy level
is 5.11 eV, while LUMO energy level is 2.90 eV. This leads to
energy band gap of 2.21 eV. In Rh-B the HOMO is located
on the positively charged nitrogen atom, while LUMO is
located throughout the molecule. The HOMO energy level
is 4.10 eV and LUMO energy level is 3.18 eV, which gives
energy band gap of 0.93 eV. Although, this value does not
correspond to red emission, it is very close to it. Thus, the
theoretically estimated values of energy levels and band
gaps are similar to those of electrochemically determined
values. When the theoretically calculated energy levels
were compared, it was found that the values decrease
gradually on moving from FL-E to PT-Hq to Rh-B. Hence this
is an indication that energy transfer might be possible
between the molecules. Thus, computational studies are
also in agreement with the possibility of FRET. Furthermore,
time dependent DFT studies (TDFT) were done to model the
absorption process. The important electronic transitions
along with the oscillator strength, wavelength and energy
are shown in Table 1.
a
b
c
Figure 12. B3LYP/6-31G* DFT calculated HOMO (bottom) and LUMO (top)
contours of (a) FL-E (b) PT-Hq and (c) Rh-B. The decrease in energy band gap
from (a) to (c) is approximate (not drawn to the scale).
Conclusions
Two conjugated D-π-A organic molecules (FL-E and PT-Hq)
were synthesized and observed to form 1D-microstructure
in the solid state suggesting the presence of strong and
efficient π-π interactions. These two molecules were mixed
with rhodamine-B (Rh-B) dye in requisite proportion to
produce white light in solution, PMMA film and gel phase.
When mixed in molar ratio of FL-E:PT-Hq:Rh-B = 1:1:1.4,
white light was obtained with CIE coordinates of 0.32 and
0.33 on excitation at 411 nm, the values being almost equal
to that for pure white light. Similarly, the CIE obtained in
film and gel form are (0.30, 0.33) and (0.31, 0.34),
respectively. The combination of all three moieties (not any
two) is crucial for white light emission as confirmed by the
control experiments. Optimum correlated colour
temperatures were also obtained in all the three cases. The
values of CCT hint towards cool white light emission in all
the three phases. The plausible mechanism for white light
generation in solution state was found to be FRET as
thoroughly studied by fluorescence titration, lifetime, and
spectral overlap integral calculations. The energy band gaps
as calculated from electrochemical studies were found to
be 2.11, 1.85 and 1.07 eV for FL-E, PT-Hq and Rh-B,
respectively. Thus, the decrease in band gaps from FL-E to
Rh-B is an indicator of the probability of energy transfer.
Furthermore, DFT studies agree with the possibility of FRET
and were also used to study the electronic distribution and
arrangements of the energy levels in all the three
molecules. It was observed that the values of energy levels
obtained from DFT studies are in agreement with those
from electrochemical experiments.
Energy
/cm-1
λ
Osc.
Energy Levels
/nm
Strength
HOMO→LUMO
(92 %)
PT-Hq
FL-E
21219
18908
471.3
528.8
1.49
0.30
HOMO→LUMO
(84 %)
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Journal Name
OC8H17
Br
S
CHO
Experimental section
Materials.
+
N
C12H25
OC8H17
9
Palladium (II) chloride [which was used to synthesise the
catalyst PdCl2(PPh3)2] and CuI were purchased from Sigma
Aldrich and Finar & Co., respectively. The compounds 3,4-
5
ethylenedioxythiophene
and
phenothiazine
were
purchased from Sigma Aldrich. Fluorene and hydroquinone
were purchased from Alfa Aesar. All these chemicals were
used as such without further purification. The solvents:
DIPA, THF, DCM and toluene were purchased from Rankem
& Co. and were distilled when necessary, using standard
procedure. The deuterated solvents were purchased from
Sigma Aldrich. Poly(methyl methacrylate) [PMMA] was
purchased from L. G. Polymers while gelatin was purchased
from S. D. fine-Chem. Pvt. Ltd.
C12H25
N
OC8H17
S
CHO
OHC
S
OC8H17
Synthesis of PT-Hq
N
C12H25
PT-Hq
PT-Hq (new) was prepared using standard Sonogashira
coupling reaction [Scheme 3]. The procedure is as follows:
In a two-necked round bottom flask, the bromo-compound
Scheme 3: Synthesis of PT-Hq (for complete numbering of the molecules
please refer to Scheme S1 of supporting information).
(9) (11.50 mmol, 2.2 eq.), Pd (II) (10 mol %), and CuI (10 mol
%) were dissolved in a 1:1 mixture of solvents (DIPA:THF)
(total 80 mL) and degassed for 15 minutes using argon.
Polymeric film and gel preparation
Then, a degassed solution of dialkyne (5) (5.23 mmol, 1 eq.)
Preparation of white light emitting PMMA film
in THF was added through a syringe to the reaction mixture
at room temperature (final solvent ratio = 1:1). Then, the
PMMA film was prepared by adding 0.5 g of PMMA to a
solution containing 1 mL of FL-E in THF, 1 mL of PT-Hq in
THF and 1.4 mL of Rh-B in water, each of initial
concentration 10-5 M. The viscous solution was kept
overnight for drying at room temperature to obtain the
desired film.
temperature was raised to 70 °C and maintained for 12 h.
After the reaction, the solvents were evaporated under
vacuum and the residue was extracted with DCM and
water, washed with brine solution, dried with Na2SO4 and
filtered. The solvent was removed under vacuum and the
resulting crude product was purified by silica gel column
chromatography (hexane/DCM 3/2) to give the compound
1
C. H NMR (400
Preparation of white light emitting gelatin gel
PT-Hq. Yield: 70 %, yellow solid. Mp. 89
MHz, CDCl3):
°
9.79 (s, 2H), 7.64 (d, J = 6.8 Hz, 2H), 7.57 (s,
δ
Gelatin gel was prepared by dissolving 0.5 g of gelatine in
10 mL of distilled water (5 % w/v). To this viscous solution,
the mixture of 1.4 mL of FL-E in THF, 1 mL of PT-Hq in THF
and 1.4 mL of Rh-B in water (each of initial concentration
10-5 M) was added. The final mixture was heated to 65
and then placed in the refrigerator, maintained at a
2H), 7.31 (d, J = 6.8 Hz, 2H), 7.25 (s, 2H), 6.96 (s, 2H), 6.89
(d, J = 6.8 Hz, 2H), 6.81 (d, J = 6.8 Hz, 2H), 4.01 (t, J = 5.2 Hz,
4H), 3.88 (t, J = 6.0 Hz, 4H), 1.79-1.85 (m, 8H), 1.52-1.69 (m,
4H), 1.37-1.43 (m, 5H), 0.87-1.35 (m, 47H), 0.86-0.88 (m,
°C,
12H). 13C NMR (125 MHz, CDCl3):
δ 190.0, 153.5, 150.0,
143.4, 131.3, 131.0, 128.5, 124.5, 123.8, 118.6, 116.7,
115.6, 115.0, 113.8, 93.9, 86.5, 69.6, 48.2, 32.0, 31.9, 29.7,
29.6, 29.5, 29.4, 29.3, 29.2, 26.8, 26.7, 26.2, 22.7, 14.2. IR
(KBr): 2924, 2857, 1682, 1581, 1357, 1469, 1413, 1200, 820,
719, 552 cm-1. UV-Vis (DCM) λmax: 417 nm. MS (MALDI) m/z:
1168.72 (100 M+). Anal. Calcd. for C76H100N2O4S2: C, 78.04;
H, 8.62; N, 2.39. Found: C, 78.35; H, 8.97; N, 2.50. All the
spectral data with assignments are given in supporting
information.
temperature of 10
gel.
°C, overnight, to get the required white
Characterization
Melting points were determined using SUNBIM (India)
melting point apparatus. 1H NMR spectra were measured
1
on a BRUKER AVANCE 400 (400 MHz for H) spectrometer.
1
The chemical shift values of H NMR are expressed in parts
per million downfield relative to the internal standard,
tetramethylsilane (δ = 0 ppm) or chloroform (δ = 7.26 ppm).
Splitting patterns are indicated as s, singlet; d, doublet; t,
triplet; q, quartet; m, multiplet. 13C NMR spectra were
measured on a BRUKER AVANCE 500 (125 MHz for 13C)
spectrometer with tetramethylsilane
(δ = 0 ppm) or
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chloroform-d (
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ARTICLE
20 L. Zhang, Y. Che and J. S. Moore, Acc. Chem. Res., 2008,
41, 1596-1608.
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Zhou, Chem. Eur. J., 2011, 17, 1660-1669.
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Interfaces, 2016, 8, 8068-8075.
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Patra, Chem. Commun., 2017, 53, 1257-1260.
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Chem. Phys., 2012, 14, 8803-8817.
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δ
= 77.0 ppm) as internal standard. Chemical
shift values are given in parts per million downfield relative
to the internal standard. UV-Visible absorption spectra
were recorded on JASCO V-630 spectrophotometer.
Fluorescence spectra and Quantum yield were measured on
FLUOROMAX 4 (Horiba Jobin Yon). Infrared spectra (IR)
were recorded on
a JASCO FTIR–4500 spectrometer.
Elemental analysis was carried out with PERKIN-ELMER
CHN-2400 analyzer. The SEM images were recorded using
an FEG Quanta 400 scanning electron microscope (electron
acceleration voltage was 3 kV). Electrochemical analyses
were carried out with a CHI 660D instrument using glassy
carbon, silver electrodes (Ag/AgCl, Ag/AgNO3) and platinum
wire as working, reference and counter electrodes,
respectively.
Acknowledgements
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This work was supported by Indian Institute of Technology,
Madras (IITM), India. Timely help from Suraj Kumar
Panigrahi in lifetime studies is heartily acknowledged.
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ARTICLE
Graphical Abstract
The appropriate composition of two new D-π-A conjugated organic molecules in combination with rhodamine B is observed to
emit cool white light in solution and solid states.
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